Metal sheets coated with resin films or coating materials are used in the form of cans for containing items such as foods and beverages. If defects occur in the films inside the cans during storage, the metal sheets are corroded on a macroscopic scale. This corrosion can lead to serious troubles such as swelling of cans due to generated gas, leakage of contents due to perforation corrosion, and food poisoning due to deterioration of contents. Thus, the evaluation of film defects in the cans is an important issue from the viewpoint of public health and also for the manufacturers of metal sheets and canned products.
The evaluation of film defects has been exclusively based on the enamel rate value method (hereinafter, abbreviated to “ERV” method). For example, Patent Literatures 1 to 3 disclose methods for evaluating the degree of metal exposure at defective film portions in coated metal sheets by the ERV method.
Further, Patent Literature 6 discloses an evaluation method, an improved version of the ERV method, which is capable of evaluating the degree and location of the exposure of base steel in tin-plated cans without being affected by tin.
Film defects occur in the inner face of cans by the three types of causes: (1) factors during the formation of films or the production of cans, (2) impacts such as when filled cans are dropped, and (3) influences of contents during long storage. The ERV method and the method disclosed in Patent Literature 6 are effective for detecting film defects ascribed to the cause (1) which occur before the filling of contents. However, these methods cannot be used for the evaluation of defects due to the types (2) and (3) which occur after the filling of contents.
As is known from Patent Literature 4, a technique is disclosed which evaluates the tendency of the occurrence of film defects in the inner face of metal cans after the cans are filled with contents. This technique is mainly developed for beverage cans, and relates to a measurement apparatus which permits film defects occurring in the inner face of cans by impacts to be measured under conditions similar to those actually experienced by canned products. In order to realize the measurement of film defects in an environment similar to that actually experienced by filled cans, the characteristic of this apparatus lies in that it is provided with a sealing function, a deaerating function, an inner pressure control function, a temperature control function, a collision device for applying an impact load to the cans, and an electrochemical measuring device for measuring the electrical resistance of the films in a sealed environment.
A similar apparatus for evaluating coated metal sheets is disclosed in Patent Literature 5.
These apparatuses measure, in particular, the degree of film defects caused by impacts (2) in the inner face of cans by making use of the electrical resistance of films.